Thunderbird's windows and pane sizes (H x W) are stored (memorised) in the file xulstore.json in the Thunderbird profile folder. Thunderbird closed, you can at least temporarily move the file xulstore.json to another location, e. g. your desktop. On restart TB will create a new xulstore.json file with default settings as if it were a new installation. Make some tests to verify if TB now memorises windows dimensions and locations.

This did not resolve the issue for Calendar Events.

I have tried resizing the Calendar Event from New Event as well as from an existing one. I immediately exit and quit the app after the resize. The xulstore.json is not updated with new width and height. When I edit the width and height manually in that file, on restart it does not recognize the new values. I think this is a bug in v141.0.

You are right,Thunderbird doesn't memorize the dimensions of the "calendar event window". Now I remember that a year ago in another TB forum someone arose the same problem and that I'd solved it with a CSS code in the userChrome.css file. The code is still in my file.

If you want to try this solution: • go to Settings> General > scroll to the bottom and click on Config Editor.... • type style in the search field, look for toolkit.legacyUserProfileCustomizations.stylesheets and toggle its value to "true" •open a text editor, copy/paste the following code in the editor

#calendar-event-window {
min-width: 55em; !important;
min-height: 85em !important;  }

and save the file as userChrome.css (delete the extension .txt your system will add to file) • create a folder, name it chrome and move the file userChrome.css in the chrome folder • open the Thunderbird profile folder and move the chrome folder in the profile folder • restart and verify the dimensions of the event window. You can adapt min-height and min-width in the code to your preferences. After a modification you must restart TB

I was hopeful that would help, but it is still not persisting the size prescribed in the CSS file.

Do you really mean the "New Event" window? My code works here in TB 141 - Mac (see picture)

So if you change that width to be even larger, it persists for you? I tried that and it did not work. I even changed it from em to px and setting static pixel size and it didn't change.

Sorry, you are right, mea culpa : I had selected the wrong element ID Try this code which should work on width as well as on height

#calendar-event-dialog {
min-width: 60em; !important;
min-height: 80em !important; 
}

Alright! Making progress now. Awesome. That worked for New Events. Now the existing calendar events are still a smaller size. Is there another element I need to amend?

I don't know what you mean by "existing calendar events". Can you post a screen shot? If I edit an existing event the window has the same size as a new event window.

Attached are a New Event and an Existing Event that I opened.

I can't recognise or reproduce the left window. Is it an invitation you accepted? If I know how to create this window I can find its identity with the developer tools.

Correct, it is a meeting invite I had previously accepted.

If I get the time I'll test with an invitation

I appreciate you helping me with these issues!

Finally I tried to reproduce your left window, to no avail. I thought it might be the "Invite Attendees" window (see picture) but this window is very different from yours. Without further information about the origin I am unable to give a code to customise your window. Maybe you should send me a dummy invitation to

mapenzi@free.fr and I will see what I can do with it.

Invite sent

Hi Sean, I received your email with the invite for this afternoon (unfortunately L.A. is too far from Paris for a meeting at 15:00 CET today. LOL !) I imported your .ics file in one of my calendars in TB, then I tried to find a window resembling yours (the left one in your message from August 13). Eventually I got a promising result: in my Calendar I had to R-click on your event and select "Open" (and not double-click the event!) This action would open a rather small box displaying an "event summary" (first picture) Using the Developer Toolbox I found the "selector ID" for this box and tested the following CSS code to customise its dimensions:

 #calendar-summary-dialog {
 min-width: 60em !important;
 min-height: 80em !important; }

With this code you can adapt the dimensions of your event summary box (second picture)

You're amazing! That was the last piece of that puzzle. Thank you so much!!!

You're welcome !